1. Field of the Invention
This invention relates to improvements in near-infrared quantitative measuring instruments and particularly, to a method and means for generating synthetic spectra for such instruments.
2. Background and Prior Art
Near-infrared quantitative measuring instruments have been available for approximately 20 years. These instruments have proven to be highly accurate and simple to use for the measurement of chemical constituents in many different types of materials. For example, near-infrared instruments are commonly used in the grain industry for determining the protein of wheat and barley, in the food industry for measuring various organic constituents within food, in the chemical process industry to determine the chemical constituents within a production product, and in the medical field for non-invasively determining such items as body fat percentage.
There are three general types of near-infrared measuring instruments. Reflectance-type instruments normally measure between 1,100 and 2,500 nanometers to provide accurate measurement of materials that have a consistent surface and require access to only one side of the product being measured. Transmission-type measurements are available that operate between 600 and 1,100 nanometers and are able to measure almost any type of product without sample preparation provided that access is available to both sides of the measured product. The third type of near-infrared instrument is the interactance type which normally operates between 600 and 1,100 nanometers. In this type of instrument, light energy is directed into a body of a product and on the same side of the body at some distance away, the internal reflected light is measured.
In any of the above-described type of near-infrared measuring instruments, the use of discrete filters or the use of full scanning instruments are known. An example of the use of a filter-type approach is shown in U.S. Pat. No. 4,286,327.
In many applications, either discrete filter or full scanning instruments will provide similar accuracy. However, there are some applications where the typical discrete filter-type instruments do not provide sufficient information. Examples of this are applications where advanced mathematical treatments such as Partial Least Square or Principle Component Analysis are applied. In such approaches, a large number of wavelengths are needed to provide the necessary calibration coefficients.
One major disadvantage of the full scanning instruments is that they are considerably more expensive than the discrete filter instruments. Thus, the desire has been to develop techniques that allow discrete filter instruments to provide the same sensitivity and versatility as full scanning instruments. One such approach is described in U.S. Pat. No. 4,627,008 where the use of curvilinear interpolation allows development of synthetic spectra from a discrete filter instrument.
However, in the measurement of very subtle constituents, e.g., non-invasive measurement of the level of glucose in the blood stream with a low cost portable instrument, accurate knowledge of spectra is required at many wavelengths. There is a need in the art to generate such spectra to provide a meaningful quantitative measuring instrument.
U.S. Pat. No. 4,286,327 teaches that a group of IREDs, each with a separate narrow bandpass filter in front of it, can be consecutively illuminated, thereby generating meaningful optical information. In such patent, a separate narrow bandpass filter is required for each wavelength to be measured. However, for a low cost portable instrument where broad spectrum information is required, it becomes essentially impractical to provide the number of narrow bandpass filters that are required. A size limitation, combined with the need for low cost, precludes such approach.
For example, research has shown that on some individuals, accurate measurement of blood glucose can be obtained by using a combination of wavelengths between 640 nanometers and 1,000 nanometers. These studies have also shown that different combinations of wavelengths are required for different individuals because of the body composition differences between people. For example, if cholesterol or glucose is desired to be measured, those constituents are in such minute quantities compared to the presence of water, fat and protein in the body that they are difficult to measure without multiple wavelengths. Thus, the need in the art exists to provide a low cost, portable, simple instrument and yet have the instrument provide the equivalent of wavelengths at every 1 nanometer between 640 to 1,000 nanometers so as to be useful over a broad population.